1. Field of the Invention
The invention relates to a pyrimidine derivative for use in the hardness stabilization of rubber-compounds and a method of improving the hardness stabilization of rubber by adding the pyrimidine derivative and associated accelerators to an unvulcanized rubber composition.
2. Discussion of the Prior Art
Vulcanizing rubber compositions by heating a sulfur-vulcanizable rubber composition with sulfur and/or a sulfur donor and a vulcanization accelerator has been known for many years. By this process vulcanizates having acceptable physical properties including tensile strength, resilience, and fatigue resistance can be obtained, but such vulcanizates tend not to have good aging properties. A typical aging phenomenon is hardening, which is explained below.
Uncured as well as cured rubbers are prone to aging effects. The unsaturated groups in diene rubbers, e.g. styrene-butadiene rubber (SBR) or a blend of SBR with natural rubber, butadiene rubber or with both, make it possible to cure with sulfur, but at the same time they exhibit a sensitivity toward oxygen, ozone, and other reactive substances causing changes such as hardening of the vulcanizate. Unaged diene rubbers contain free double bonds that remain sensitive to the above reactive substances even after vulcanization. Higher temperatures make these effects even more noticeable. Also, since unreacted double bonds are present in the rubber vulcanizate, there is the possibility of further reaction with sulfur causing hardening, i.e. additional crosslinking, of the vulcanizate.
The use of antioxidants will retard the oxygen-induced aging of the vulcanizate, but will not affect the increase in hardness due to sulfur-induced crosslinking.
L. H. Davis et al. in Rubber Chemistry and Technology, Vol. 60, 1987, 125-139, disclose the use of 2,2xe2x80x2-dithiobispyridine-N-oxide and the zinc salt of pyridine-2-thiol-N-oxide as a primary accelerator alone or in combination with a low amount of a benzothiazole-2-sulfenamide accelerator in the sulfur vulcanization of polyisoprene, e.g., natural, rubber compounds.
U.S. Pat. No. 3,574,213 discloses rubber vulcanization accelerators comprising pyrimidinylthio-phthalazines, particularly 1-(4,6-dimethyl-2-pyrimidinylthio)-phthalazine, that achieve reduction in scorch.
C. J. Rostek et al, in Rubber Chemistry and Technology, Vol. 69, 1996, 180-202, disclose the use of novel sulfur vulcanization accelerators based on mercapto-pyridine, -pyrazine, and -pyrimidine. This reference relates to polyisoprene rubbers, which do not harden.
U.S. Pat. No. 3,839,303 discloses the inhibition of premature vulcanization of natural or synthetic diene rubbers by including in the vulcanizable composition accelerating agents, such as thiazole accelerators and a compound comprising certain pyrimidinesulfenamides, such as N-cyclohexyl-4,6-dimethyl-2-pyrimidinesulfenamide. The compound of this reference is formulated so as to be effective in inhibiting premature vulcanization in the vulcanizable composition to which it is added.
In one embodiment, the present invention comprises a vulcanizable composition comprising a sulfur vulcanizable rubber, a sulfur vulcanizing agent, an accelerating agent selected from the group consisting of sulfenamide accelerators, other than thiazole sulfenamides, and a hardness stabilization agent comprising a pyrimidine derivative of the formula: 
Where X=H, R1 through R4, NR3R4, OR5, SR5, SO2R6, M, (SO3),M (M=metal ion), and n and z can be the same or different and 1, 2 or 3, depending on whether the respective valence of X and M is 1, 2 or 3, R1 through R4 are the same or different and selected from the group consisting of the substituents H, halogen, OH, NH2, alkyl, cycloalkyl, aryl, alkylaryl, aralkyl, with the substituents alkyl, cycloalkyl, aryl, alkylaryl and aralkyl optionally having additional functional groups selected from the group consisting of NH2, OH, substituted amino, substituted hydroxyl, halogen, and carbonyl containing group, when R3 and/or R4 are one of the substituents alkyl, cycloalkyl, aryl, alkylaryl and aralkyl, they may be in the same constituent in a ring together with N to form a heterocyclic group, R5 is selected from the group consisting of the substituents H, alkyl, cycloalkyl, aryl, alkylaryl and aralkyl, or a radical derived from a carbon based heterocyclic group containing at least one of S or N, or both S and N, R6 is selected from the group consisting of the substituents alkyl, cycloalkyl, aryl, alkylaryl, aralkyl, H, OH, OM, NH2, NR3R4, and OR5, the amount of accelerating agent in said composition being greater than about 0.6 phr when said rubber is SBR and at least about 0.5 phr when said rubber is natural rubber, and the amount of hardness stabilization agent being at least about 0.5 phr.
In a second embodiment, the present invention comprises a method of improving the hardness stabilization of rubber which includes adding the above composition to an unvulcanized rubber composition followed by vulcanization of the rubber composition.
Other embodiments of the invention encompass specific pyrimidine derivatives, accelerators, details about relative amounts of reactants, and unvulcanized rubber compositions, all of which are hereinafter disclosed in the following discussion of each of the facets of the present invention.
According to the present invention, it has been found that by adding appropriate amounts of certain pyrimidine derivatives and non-thiazole sulfenamide accelerators to a vulcanizable rubber composition comprising natural rubber or other rubbers, vulcanizates, from which, e.g., pneumatic tires can be made, having improved properties can be obtained. These combinations of accelerators and pyrimidine derivatives have the effect of stabilizing the hardness properties of the rubber vulcanizate, e.g., during the service life of a pneumatic fire, without inhibiting or slowing vulcanization, i.e. increasing xe2x80x9cscorchxe2x80x9d time, in the production of the tire. Thus, hardness stabilization is achieved without slowing of the vulcanization process, thereby avoiding loss in production efficiency.
In this application, the abbreviation xe2x80x9cphrxe2x80x9d, means the number of parts by weight per 100 parts by weight of rubber. In the case of a rubber blend, it is based on 100 parts by weight of total rubber.
Either natural rubber (NR), styrene-butadiene rubber (SBR) or a blend of NR and SBR or NR and SBR with one or more other rubbers can be used in the invention process, it being understood that for purposes of this invention the term xe2x80x9crubberxe2x80x9d means; an elastomer containing a hydrocarbon unit which is a polymer with some unsaturated chemical bonds. Typically, SBR, a blend of SBR with natural rubber (NR), a blend of SBR with polybutadiene rubber or butadiene rubber (BR), or a blend of SBR with NR and BR is used. The type of rubber or mixture of rubbers will have some affect on the precise amounts of accelerator and pyrimidine derivative appropriate to achieve hardness stabilization without inhibition of the vulcanization.
Typically, the amount of pyrimidine derivative hardness stabilizing agent employed in the process of the present invention will be at least about 0.5 phr. The preferred upper limit is about 10.0 phr, most preferably 3.0 phr.
In the process of the present invention sulfur and/or a sulfur vulcanizing agent is employed. The amount of sulfur to be compounded with the rubber usually is 0.1 to 10 phr, preferably in excess of about 1 phr. If a sulfur donor is used the amount thereof should be calculated in terms of the amount of sulfur.
Typical examples of sulfur donors that can be used in the process of the present invention include dithiodimorpholine, caprolactam disulfide, tetramethylthiuram disulfide, and dipentamethylenethiuram tetrasulfide. The reader is referred to W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989, in particular pages 231-233.
Particularly preferred pyrimidine derivatives for use in the composition and method of the present invention have the following chemical structural formulas:
2-Benzothiazoyl-4,6-dimethyl-2-pyrimidyl disulfide 
N-Cyclohexyl 416-dimethyl-2-pyrimidinesulfenamide 
S-(4,6-Dimethyl-2-pyrimidyl) p-toluenethiosulfonate 
It is preferred that the alkyl, cycloalkyl, aryl , alkyl aryl and aralkyl groups of the above formula I have from 2 to about 15 carbon atoms and most preferably 2 to about 8 carbon atoms.
Preferred radicals derived from a heterocyclic group for R5 of formula I are 2-benzothiazoyl and a pyrimidine.
A further preferred pyrimidine derivative of formula I for use in the composition and method of the present invention is 2,2xe2x80x2 bis 4,6-dimethylpyrimidyl disulfide.
The preferred metal that may be in the compound of formula I is selected from the group consisting of Zn, Ni, Mg, Co and Na.
The preferred carbonyl containing groups of formula I include carboxylic acid or its salt, ester, amide, ketone or aldehyde.
Typical vulcanization non-thiazole sulfenamide accelerating agents (accelerators) appropriate for use in the invention include benzothiazole-based accelerators, particularly mercaptobenzothiazoles, thiophosphoric acid derivatives, thiurams, dithiocarbamates, diphenylguanidine (DPG), di-o-tolyl guanidine, xanthates, and mixtures of one or more of these accelerators. Preferably, the vulcanization accelerator comprises mercaptobenzothiazoles, most preferably 2-mercaptobenzothiazole (MBT). A particularly preferred accelerating agent is bis(dibenzylthiocarbamoyl)disulfide.
Particularly effective sulfur-vulcanizable rubber compositions in accordance with the present invention include a composition comprising styrene-butadiene rubber, 2-pyrimidinesulfenamide, and a mixture of the accelerators, bis(dibenzylthiocarbamoyl) disulfide and 2-mercaptobenzothiazole, or a composition comprising natural rubber, 2-pyrimidinesulfenamide and the accelerator, 2-mercaptobenzothiazole.
We have found that with SBR, the amount of accelerator should be greater than about 0.6 phr of accelerator, and for natural rubber at least about 0.5 phr of accelerator, with upper limits in either case preferably about 10.0 phr and most preferably about 3.0. Natural rubber has more reactive allylic sites for crosslinking than SBR and generally requires less accelerator for efficient crosslinking.
It may be effective, in lieu of directly providing a pyrimidine derivative of formula I in the composition of the invention, to provide precursors of such derivatives. It is particularly preferred to use thiazole derivatives and 2-mercaptopyrimidines that lead to the formation of such derivatives, in situ. A derivative for which in situ formation is particularly useful is 2-benzothiazoyl-4,6dimethyl-2-pyrimidyl disulfide.
Conventional rubber additives may also be included in the sulfur-vulcanizable rubber composition in accordance with the present invention. Examples include reinforcing agents such as carbon black, silica, clay, whiting and other mineral fillers, processing oils, tackifiers, waxes, phenolic antioxidants, phenylenediamine antidegradants, antiozonants, pigments, e.g. titanium dioxide, resins, plasticizers, factices, and vulcanization activators, such as stearic acid and zinc oxide. These conventional rubber additives may be added in amounts known to the person skilled in the art of rubber compounding. The reader is also referred to the examples that are described below.
For further details on these typical rubber additives and vulcanization inhibitors, see W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989.
Finally, in specific applications it may also be desirable to include steel-cord adhesion promoters such as cobalt salts and dithiosulfates in conventional, known quantities.
The sulfur vulcanization process of the present invention can be carried out using means and equipment that are well known to a person skilled in the art. Suitable vulcanization procedures are described in W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989.
A typical method comprises preparing a masterbatch consisting of rubber, carbon black, a vulcanization activator, and a processing oil, in an internal mixer such as a Banbury mixer or a Wemer and Pfleiderer mixer, and subsequently adding a vulcanization system comprising sulfur and a vulcanization accelerator, and the hardness stabilizing pyrimidine derivative in accordance with the present invention to the masterbatch either in a low temperature mixer or on a two-roll mill, i.e. the productive stage of mixing. The uncured rubber composition is then vulcanized by heating, e.g., in a compression mold.
The invention vulcanization process typically is carried out at a temperature of 110-200, preferably 120-190, more preferably 140-180xc2x0 C. for a period of time of up to 12, preferably up to 6, more preferably up to 1 hour.
The composition of the present invention is useful in the. manufacture of many articles, including pneumatic tires, e.g., for passenger cars and trucks, and industrial rubber goods, which comprise the rubber vulcanizate obtained by the method of the invention.